Method and apparatus for operating a vacuum interface of a mass spectrometer

Abstract

Method for operating a vacuum interface (3) of a mass spectrometer (10), wherein the vacuum interface (3) comprises an evacuated expansion chamber downstream of a plasma ion source (6), the plasma ion source (6) being located at a pressure between atmospheric pressure and 100 mbar, the expansion chamber having a first aperture interface with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump (40) to provide an interface pressure in the expansion chamber; wherein the method comprises the use of a controller (50) for automatically controlling the throughput of the interface vacuum pump (40) to control the interface pressure depending on operating modes of the mass spectrometer (10), wherein the throughput of the interface vacuum pump (40) is controlled by changing the pump speed by varying the operating voltage and / or the operating current of the interface vacuum pump (40), and the throughput of the interface vacuum pump (40) is controlled depending on one or more operating conditions of the plasma ion source (6) and / or one or more elements of interest to be mass-analyzed by the mass spectrometer (10), wherein the controller (50) is connected to a visual display unit (VDU) so that the initially automatically controlled flow rate of the interface vacuum pump (40) and / or the interface pressure is displayed to a user, and wherein the user can automatically Control (50) via a graphical user interface (GUI) displayed on the VDU and an input device (72) overrides and manually sets a specific throughput of the vacuum pump (40) and / or a specific interface pressure.

Description

Field of invention

[0001] The invention relates to the field of mass spectrometry and, in particular, to a method and an apparatus for operating a vacuum interface, especially, but not exclusively, an atmosphere-vacuum interface of a mass spectrometer. The method and the apparatus are suitable for use primarily with a plasma ion source, such as an inductively coupled plasma (ICP), microwave-induced plasma (MIP), or laser-induced plasma ion source. For illustrative purposes, the following description focuses on embodiments using inductively coupled plasma mass spectrometry (ICP-MS). background

[0002] The general principles of ICP-MS are well established. ICP-MS instruments offer robust and highly sensitive elemental analysis of samples down to the parts per trillion (ppt) range and beyond. Typically, the sample is a liquid solution or suspension and is introduced into the plasma via an atomizer as an aerosol in a carrier gas, generally argon or sometimes helium. The nebulized sample enters a plasma torch, which typically comprises several concentric tubes forming corresponding channels and is surrounded at the downstream end by a spiral induction coil. A plasma gas, typically argon, flows into the outer channel, and an electrical discharge is applied to ionize a portion of the plasma gas.A high-frequency (HF) electric current is supplied to the spiral burner coil, and the resulting alternating magnetic field accelerates the free electrons, causing further ionization of the plasma gas. This process continues until a stable plasma state is reached at temperatures typically between 5,000 K and 10,000 K. The carrier gas and the nebulized sample flow through the central burner channel and enter the central region of the plasma, where the temperature is high enough to cause atomization and subsequent ionization of the sample.The sample ions in the plasma must next be shaped into an ion beam for ion separation and detection by the mass spectrometer, which can be provided by, among others, a quadrupole mass analyzer, a mass analyzer with a magnetic sector and / or electric sector, a time-of-flight mass analyzer, or an ion trap mass analyzer.

[0003] Thus, in ICP-MS, ions are formed under atmospheric pressure or relatively high pressure (e.g., above 100 mbar) outside the main vacuum system of the spectrometer. For most mass analyzers, a vacuum with a pressure of <5 x 10⁻⁶ m / s is required. -5 mbar is required. Therefore, an interface region is provided that regulates the transfer from the atmospheric pressure ion source to the high-vacuum mass analyzer (see Fig.1, which is described below). This typically comprises several stages of pressure reduction, ion extraction from the plasma, and ion beam generation, and may include a collision / reaction cell stage to remove potentially interfering ions from the mass analysis. The first stage of pressure reduction is achieved by sampling the plasma through a first aperture in a vacuum interface, typically provided by a sampling cone with an aperture-equipped tip, typically having an inner diameter in the range of 0.5 to 1.5 mm. The sampling cone is the typical component that is connected to the plasma source at atmospheric or relatively high (>100 mbar) pressure. The sampled plasma expands downstream of the first aperture into an evacuated expansion chamber, where the pressure is typically a few mbar (e.g., 1 to 10 mbar).The central portion of the expanding plasma then passes through a second aperture, typically provided by a skimmer cone, into a second evacuation chamber, which has a higher degree of vacuum than the expansion chamber. As the plasma expands through the skimmer cone, its density decreases sufficiently to allow the extraction of ions to form an ion beam using strong electric fields generated by ion lenses downstream of the skimmer cone. The resulting ion beam can be deflected and / or directed to the mass spectrometer by one or more ion deflectors, ion lenses, and / or ion conductors, which can operate with static or time-varying fields.

[0004] A collision / reaction cell can be provided upstream of the mass spectrometer to remove potentially interfering ions from the ion beam. These are typically argon-based ions (such as Ar+, Ar+). 2+ , ArO + ), but can also include others, such as ionized hydrocarbons, metal oxides, or metal hydroxides. The collision / reaction cell promotes ion-neutral collisions / reactions, whereby the unwanted molecular ions (and Ar) are removed. + , Ar 2+The analyte ions are preferably neutralized and pumped out along with other neutral gas components, or dissociated into ions with lower mass-to-charge ratios (m / z) and discarded in a downstream m / z discrimination (mass filter) stage. Alternatively, the analyte ions can preferably be subjected to mass-shift reactions so that the resulting mass-shifted ions can be separated from the interfering ions in a downstream m / z discrimination stage. US 7,230,232 B2 and US 7,119,330 B2 provide examples of collision / reaction cells used in ICP-MS.

[0005] The ICP-MS instrument should preferably meet a number of analytical requirements, including high transmission, high stability, and low influence of the sample matrix (the mass composition of the sample, including, for example, water, organic compounds, acids, dissolved solids, and salts) in the plasma, and low throughput of oxide ions or doubly charged ions, etc. These parameters can depend strongly on the interface properties.

[0006] The interface properties are influenced by various processes, such as particle and gas dynamics, any secondary discharges, and the kinetic energies of the charged species passing through the interface (including doubly charged oxides and hydroxide species). Changes in the plasma and / or at the interface, such as altering the sampling components (e.g., the inner diameter of the sampling aperture and skimmer cone), measuring under hot or cold plasma conditions, and using organic solvents, directly affect the interface properties and thus the ion transport efficiency.

[0007] Referring to Fig.1. A pre-vacuum pump 40, also known as a pretreatment pump, is normally used to evacuate the interface 3 between the sampling cone and the skimmer cone 2 and 12. The sampling cone 2 is used to sample the atmospheric pressure plasma flame 4 extending from the end of the plasma torch 6. The sampling cone 2 has a central aperture 8 with an inner diameter of approximately 1 mm, which typically allows a pressure P1 of approximately 1–5 mbar in the interface region 3 between the sampling cone 2 and the skimmer cone 12, for example, when a pumping rate of 5–15 l / s is used. These conditions determine the correct position of the aperture of the skimmer 12 within the so-called dead zone of the extracted plasma, which is defined by the dotted line 14 in the figure. Fig.Figure 1 illustrates this. A typical distance between the tips of the sampling and skimmer cones can be approximately 10 mm. The extracted beam forms a concentric shock wave structure that terminates in a shock wave front referred to as the Mach disk 16. The region within this shock wave structure is called the dead zone 14 and contains the ions, electrons, and neutral particles from which the ions must be transferred to the mass analyzer. This means that the skimmer cone aperture should be positioned within the dead zone for adequate extraction of the analyte ions. In other words, if the sampling and skimmer cones are static, as is generally the case, the interface pressure in region 3 must be low enough to position the Mach disk behind the skimmer aperture [see Inductively Coupled Plasma Mass Spectrometry, Akbar Montaser, John Wiley & Sons, 1998, ISBN 0471186201, 9780471186205].The correlation between machining disc and interface pressure was described by Olney et al. [Olney et al., J. Anal. At. Spectrom., 1999, 14, 9-17] as:. xMDo=0.67P0P1 where: x M the distance between sampler and Mach disk is D o the diameter of the sampler opening is P0 is the source pressure (atmospheric pressure). P1 the interface pressure is

[0008] Chiappini et al., Development of a high-sensitivity inductively coupled plasma mass spectrometer for actinide measurement in the femtogram range, J. Analytical Atomic Spectrometry, 1996, 11, 497-503, demonstrated that reducing the interface pressure can improve the instrument's sensitivity. In this case, the lower pressure was achieved by adding an additional pump to the vacuum system.

[0009] Beyond interface 3, the plasma is exposed to an ion extraction field via an ion extraction optic 20. This field attracts positive ions from the plasma into an ion beam, repels electrons, and enables the removal of neutral components. The ion beam is then transported downstream via an ion optic (not shown) to a mass analyzer 30 (not shown in detail) for mass analysis. The ion beam can be deflected by the extraction optic 20 and / or guided towards the mass analyzer 30 by one or more ion deflectors, ion lenses, and / or ion conductors (not shown), which can operate with static or time-varying fields.A collision / reaction cell can be located upstream of the mass analyzer, optionally with a mass filter located upstream of the collision / reaction cell (the latter configuration can be provided by a triple quadrupole arrangement as in the Thermo Scientific™ iCAP™ TQ ICP-MS).

[0010] However, the basic principles of the dead zone and the shock wave description of the plasma outlined above do not fully explain the influence of the interface pressure, and there remains a need to improve the sensitivity of the instrument in ICP-MS under both routine and specific experimental research conditions. Against this background, the present invention was developed.

[0011] US patent US 6,265,717 B1 discloses an ICP-MS device with an interface for transferring ions from the ICP to the MS. The interface is equipped with a control system for increasing the pressure at the interface from its normal pressure to selectively reduce interfering ions. A variable valve in the pump line can be controlled by a system controller connected to a personal computer.

[0012] Japanese patent application JP H11-185695 A discloses an ICP-MS device equipped with a variable valve for pressure regulation in an interface chamber, enabling operation with both hot and cold plasma. The method of controlling the variable valve is not disclosed. Brief description

[0013] According to one aspect of the invention, a method for operating a vacuum interface according to claim 1 is provided. This makes it possible to optimize the detection sensitivity of the spectrometer, i.e., detection limits, for one or more elements that are subjected to mass analysis by the mass spectrometer. The control of the vacuum pump flow rate preferably depends on one or more operating modes of the spectrometer; that is, the method preferably comprises automatically adjusting the flow rate depending on one or more operating modes of the spectrometer. Preferably, the vacuum pump flow rate is automatically controlled depending on one or more operating conditions of the plasma ion source and / or one or more elements of interest that are to be mass-analyzed by the spectrometer.Preferably, the vacuum pump is controlled directly, for example by varying its operating voltage and / or operating current to vary its speed, rather than indirectly, as is the case when using a valve.

[0014] According to another aspect of the invention, a device for operating a mass spectrometer vacuum interface according to claim 13 is provided.

[0015] The controller is preferably configured to automatically control the vacuum pump flow rate depending on one or more operating modes of the spectrometer. The operating modes may include a plasma condition and / or a measurement mode (e.g., analysis of a specific element). The controller is preferably configured to automatically control the vacuum pump flow rate depending on one or more operating conditions of the plasma ion source and / or one or more elements of interest to be mass-analyzed by the spectrometer (measurement modes).

[0016] The one or more operating conditions of the plasma ion source preferably include the plasma temperature (i.e., as determined by the power supplied to the plasma ion source), a plasma torch position, and / or a plasma gas flow. The identification of the one or more elements of interest to be mass-analyzed by the spectrometer can be entered into the controller by a user, enabling the controller to automatically regulate the vacuum pump flow rate to optimize the detection sensitivity for the one or more elements.

[0017] The invention thus enables the interface pressure to be regulated by a control system, i.e., controlled to optimize the detection sensitivity of the spectrometer for a specific element or elements being analyzed under a given operating mode or set of operating conditions (e.g., the power supplied to the plasma / plasma temperature). In particular, the invention can ensure that the optimal interface pressure is provided by regulating the flow rate or pump speed of the interface vacuum pump to achieve the best sensitivity of the instrument under various experimental conditions (e.g., hot / cold plasma). Such regulation of the interface pressure depending on the characteristics of different measurement modes is not provided in commercial ICP-MS instruments.Furthermore, direct control of the interface vacuum pump is more efficient than regulating the interface vacuum via a valve or other mechanism.

[0018] The invention further relates to a method for operating a vacuum interface according to claim 25 and a device for operating a mass spectrometer vacuum interface according to claim 27. Brief description of the drawings Fig. Figure 1 shows a schematic view of a vacuum interface region. Fig. Figure 2 shows a schematic view of a mass spectrometer. Fig. Figure 3 shows bar graphs of the intensities of selected elements measured on an ICP-MS instrument using different settings for the rotational speed of the interface vacuum pump for both hot (top graph) and cold (bottom graph) plasma conditions. Detailed description

[0019] To enable a more detailed understanding of the invention, various embodiments will now be described.

[0020] Referring to Fig. Figure 2 shows a mass spectrometer 10 according to an embodiment of the present invention. The mass spectrometer comprises a vacuum interface, as is generally the case in Fig.Figure 1 illustrates the spectrometer. It comprises three vacuum stages: the interface vacuum 3, an intermediate vacuum 5, and a high vacuum 7. The inductively coupled plasma torch (ICP) 6, as described above, generates a high-temperature plasma at atmospheric pressure from a gas such as argon. It receives a sample containing one or more elements to be analyzed and ionizes the sample within the plasma. Generally, the plasma ion source can be an ICP, a MIP, a laser-induced plasma, or another type of plasma ion source. Thus, the plasma ion source generates element ions that can be mass-analyzed in the downstream mass analyzer. The operation of a plasma ion source and the introduction of a sample into it are described above.Although the plasma generally operates at atmospheric pressure, it can be at a lower pressure than atmospheric pressure, but at least at a relatively high pressure (typically at least 100 mbar) compared to the interface pressure in the expansion chamber. Thus, the plasma ion source generally operates at a pressure greater than 100 mbar, typically at atmospheric pressure. The ICP conditions can be varied between hot (e.g., 1550 W ICP torch power) and cold (e.g., 550 W), with an optional warm setting between the hot and cold settings.

[0021] The plasma, containing ions, gas, and electrons, is sampled through a first aperture provided by an aperture sampling cone 2 and forms an expanding plasma downstream of it in an expansion chamber, as shown in Fig.Figure 1 shows the process. The plasma is then skimmed through a second aperture provided by a skimmer cone 12 and forms a skimmed or secondary plasma expansion downstream of it. An expansion chamber, forming the interface vacuum stage 3, is located between the sampling cone 2 and the skimmer cone 12, with the pressure typically in the range of 0.1 to 10 mbar, preferably 1 to 10 mbar.

[0022] Beyond the interface vacuum stage 3, the skimmed plasma is exposed to an ion extraction field by the ion extraction optics 20 located in the intermediate vacuum stage 5. This field attracts positive ions from the plasma into an ion beam, repels electrons, and allows the removal of neutral components. The intermediate vacuum stage 5 is typically set to approximately 1 × 10 -5 up to 5×10 -5The ion beam is pumped to a pressure of mbar. In the intermediate vacuum stage 5, it is then transported downstream through 90-degree bending ion optics 22 to a gas-filled collision / reaction cell 24, which includes a quadrupole for interference removal. Beyond the intermediate vacuum stage 5, the ion beam is finally guided to a quadrupole mass analyzer 30 and an ion detector 32, such as a SEM, located in the high vacuum stage 7, where the pressure is generally lower than in the intermediate vacuum stage 5, and thus typically less than 1–5 × 10⁻⁶ mbar. -5The pressure is in mbar. Optionally, a mass pre-filter, such as a quadrupole mass filter, can be arranged upstream of the collision / reaction cell 24 to improve the removal of interference from the ion beam. The latter configuration therefore provides a triple quadrupole array. It is understood that other types of mass analyzers can be provided as an alternative to the quadrupole mass analyzer, such as, among others, a mass analyzer for the magnetic and / or electrical sector, a time-of-flight mass analyzer, or an ion trap mass analyzer.

[0023] The expansion chamber of interface vacuum stage 3 is typically pumped to a pressure in the range of 1 to 10 mbar by an interface vacuum pump 40. The interface vacuum pump is preferably a backing pump, also referred to as a pretreatment pump. The intermediate and high vacuum stages 5 and 7 are pumped by a partial-flow turbomolecular pump (not shown), the exhaust of which is pumped by the backing pump 40. Thus, in some embodiments, the backing pump is advantageously the backing pump for a high-vacuum pump that pumps a high-vacuum region of the mass spectrometer.

[0024] The regulation of the flow rate (or speed) of the vacuum pump preferably comprises automatic control of the vacuum pump's flow rate using a controller 50 that interfaces with the pump. The controller can be a computer-based controller. The controller 50 can include a computer and associated control electronics, for example, a control voltage supply, which interfaces with the vacuum pump 40. The flow rate of the vacuum pump 40 can be regulated by the controller, for example, via digital connections on the pump that interface with the controller, and using software running on the controller's computer. In this way, the pump's speed can be controlled, for example, via an external voltage to control the settings of the pump's frequency converter.The vacuum pump is preferably configured such that its throughput can be continuously or quasi-continuously (e.g., essentially continuously and / or at certain intervals) adjusted by the controller over a range of throughput rates (measured, for example, in pumping rate (L / s) or pumping cycles per second (Hz)), i.e., the pump has more than just two or three individual pumping rates that can be set, but effectively a multiple of this number due to its continuously adjustable speed.

[0025] The interface vacuum pump is preferably a backing pump, such as a vane, scroll, rotary lobe, or diaphragm pump, particularly oil-free models of such pumps. Therefore, the interface vacuum pump is preferably a dry backing pump. Such pumps are preferably capable of achieving a pressure in the range of 0.1 to 100 mbar. The interface pressure of the expansion chamber is typically designed for the range of 1 to 10 mbar. The flow rate or pumping speed of the interface vacuum pump is preferably controlled by changing the pump speed, for example, by changing the speed of a vane or rotary lobe pump. Alternatively or additionally, in some embodiments, the flow rate of the backing pump can be controlled by controlling the size of an aperture located on the inlet side (i.e., upstream) of the pump.For this purpose, an electromechanically variable aperture can be used, e.g. in a proportional valve that has an interface with the control system.

[0026] The controller can, for example, also control the operation of the mass spectrometer 10 via suitable control electronics, including one or more plasma ion sources, ion optics, collision cells, and mass analyzers. The controller's computer can also acquire and process data from the mass spectrometer, in particular signals from the mass analyzer and detector 32, e.g., to generate a mass spectrum from the mass analysis. In some embodiments, the data acquired by the mass spectrometer can be used by the controller, optionally after processing, to adjust the throughput of the vacuum pump in order to optimize the detection sensitivity of the spectrometer for at least one element.

[0027] In some embodiments, the flow rate of the vacuum pump 40 is automatically controlled depending on the operating conditions of the plasma ion source 6. For example, it is known to operate a plasma ion source of a mass spectrometer either under hot plasma conditions, for example with an ICP ion source power of 1300 to 1700 W (resulting in a plasma temperature of, for example, 8,000 to 10,000 K), or under cold plasma conditions, for example with an ICP ion source power of 400 to 600 W (resulting in a plasma temperature of, for example, approximately 5,000 K). The choice of hot or cold plasma can depend, for example, on the specific elements in the sample to be subjected to mass analysis and the sample matrix. Cold plasma for ICP-MS has advantages for the ultra-low detection of certain trace elements.At low RF power, the overall ionization efficiency in ICP is reduced, thereby limiting the formation of background argon and some sample matrix-based interferences. This reduces the need for operation in additional interference reduction modes, thus reducing analysis time and improving sample throughput. When the ICP ion source operates in cold mode at a lower RF power (generally <600 W), the plasma is smaller, and any interaction with the sampling interface and sample introduction system is reduced. For elements with low ionization potentials (such as Li and Na, etc.), a preferentially higher signal-to-noise ratio can be achieved in cold plasma than in hot plasma, resulting in lower limits of detection.

[0028] However, a change in plasma conditions can lead to a change in the interface pressure for a given backing pump throughput, and consequently to interface properties that result in suboptimal ion transfer and detection sensitivity.

[0029] To address this problem, embodiments of the invention may include changing the operating conditions of the plasma ion source from a first operating condition to a second operating condition or vice versa, and in each case automatically adjusting the flow rate of the interface vacuum pump from a first flow rate when operating the plasma under the first operating condition to a second flow rate when operating the plasma under the second operating condition. The first and second flow rates preferably differ from each other. The controller 50 preferably controls the operating conditions of the plasma ion source 6, for example according to a selection of an operating condition (e.g., the plasma temperature or power supplied to the ICP torch), which is entered, for example, by a user, and automatically adjusts the flow rate of the interface vacuum pump 40 depending on the conditions, e.g.,according to a computer program (software) running on the computer of the controller 50. The controller 50 can automatically adjust the throughput of the interface vacuum pump 40 by varying the supply voltage and / or the supply current of the pump, for example, the amplitude of the supply voltage and / or, in the case of an AC pump, the frequency of the supply voltage.

[0030] In a preferred embodiment, the first and second operating conditions differ in plasma temperature; preferably, the first operating condition can be a hot plasma condition and the second operating condition can be a cold plasma condition. For example, the vacuum pump throughput can be increased when switching from the hot plasma condition to the cold plasma condition, for example by increasing the speed of the vacuum pump 40. Other different operating conditions can relate to standby (plasma off), different plasma torch positions, and / or different plasma gas flows.

[0031] With reference to Fig. 3 are the intensities of selected elements ( 7 Li, 59 Co, 115 In, 209 Bi, 238U) in an iCAP TQ tuning solution (1 ppb Li, Co, In, Ba, Ce, Bi, U in 2% HNO3), measured with a Thermo Scientific™-iCAP™ TQ ICP-MS using different backing pump speed settings, resulting in different interface pressure readings as measured with a Pirani gauge. The backing pump was a Leybold Oerlikon ecoDry 65plus rotary lobe pump. The upper bar graph in Fig. Figure 3 shows the results of a test using hot plasma conditions (ICP power 1550 W). The lower bar chart shows the results of a test using cold plasma conditions (ICP power 550 W). Table 1 shows the pump speeds (in pump cycles or revolutions per second, Hz) and the corresponding pressures obtained at the vacuum interface under the hot and cold conditions. Table 1 Pump speed (Hz) Interface pressure (mbar) HOT plasma Interface pressure (mbar) COLD plasma 100 3,323 3,803 120 1,980 2,371 130 1,769 2,118 140 1,654 1,980 160 1,511 1,769 180 1,351 1,654 200 1,320 1,581

[0032] First, it is evident that different interface pressures are obtained under hot and cold plasma conditions at the same pump speed. For example, a Pirani pressure of 1.769 mbar is achieved with a pump speed of 130 Hz under hot plasma conditions, while only a vacuum pressure of 2.118 mbar is achieved under cold plasma conditions. Accordingly, when selecting cold plasma, the pump speed must be increased from 130 Hz to 160 Hz to achieve a similar interface pressure of 1.769 mbar. The advantage of increasing the pump speed under cold plasma conditions is evident from the Li intensities, which increase by a factor of 2 when the pump speed is increased from 130 to 160 Hz.Secondly, it is evident that the instrument's detection sensitivity for each element depends on the interface pressure, and furthermore, the sensitivity behavior for different elements does not exhibit the same pattern of interface pressure dependence for all elements. Thus, it is clear that an optimal interface pressure can be set by the controller through appropriate control of the vacuum pump flow rate, providing optimal detection sensitivity for a given element. Maintaining the interface pressure at this optimum during mass analysis of that element on the spectrometer can be achieved by means of a pressure gauge signal, which is fed back to the controller as described below.

[0033] The first and second throughputs of the vacuum pump are preferably set by the controller 50. More precisely, the first throughput is preferably set by the controller, preferably optimized to provide a first interface pressure in the interface vacuum stage 3. Preferably, the first interface pressure optimizes the detection sensitivity for at least one element of the sample that is mass-analyzed using the first operating condition. The second throughput is preferably set by the controller, preferably optimized to provide a second interface pressure. Preferably, the second interface pressure optimizes the detection sensitivity for at least one element that is mass-analyzed using the second operating condition.The at least one element whose detection sensitivity is optimized under the first operating condition can be the same as, but preferably different from, the at least one element whose detection sensitivity is optimized under the second operating condition. In some embodiments, it is advantageous for the control system to regulate the first and second interface pressures so that they are substantially equal (preferably the same pressure within 10% or, more preferably, within 5%, i.e., the lower pressure is within 10% or 5% of the respective higher pressure of the two). It is understood that in this way, the interface pressure in the expansion chamber is kept substantially constant by adjusting the flow rate of the vacuum pump after changing the plasma conditions. It is understood that operating conditions other than those with hot and cold plasma, e.g.,Sample depth, cone openings or sample matrix (organic, aqueous) could also be changed to provide the first and second operating conditions and the throughput of the vacuum pump adjusted accordingly by the controller, e.g. to maintain the essentially constant interface pressure in the expansion chamber.

[0034] It is understood that in some embodiments more than two different operating conditions can be provided, e.g., set by the controller 50, each having a respective vacuum pump flow rate set by the controller. Thus, in addition to the first and second operating conditions and flow rates described, one or more further operating conditions and one or more further flow rates can be used analogously. For example, a third operating condition of the plasma ion source, such as a "warm" plasma condition, whose power and thus temperature lie between that of hot and cold conditions, can be used, with the controller setting a corresponding third vacuum pump flow rate to provide a third interface pressure.The third interface pressure can be essentially the same as, or different from, the first and second interface pressures (preferably different). As another example, a further operating condition of the plasma ion source can be a shutdown or standby condition (i.e., the plasma is switched off), with the controller then adjusting the vacuum pump's flow rate, specifically the pump speed, to provide another interface pressure. Generally, when the operating condition is a shutdown or standby condition, the corresponding vacuum pump flow rate (speed) is lower than for other operating conditions (e.g., when the analysis is performed with the spectrometer) to provide a lower vacuum (higher pressure) for the interface pressures in the expansion chamber.In this way, operating the vacuum pump at a lower speed in standby mode provides reduced power consumption or an "Eco" mode.

[0035] In some preferred embodiments, a pressure gauge 60, e.g., a Pirani pressure gauge, may be located in the expansion chamber of the interface vacuum stage. During operation, controlling the flow rate of the vacuum pump 40 may involve: providing a set pressure value to the controller 50, which may be entered by the user (e.g., via a graphical user interface) or provided in the controller's computer software based on the operating conditions; receiving pressure signals at the controller 50 over time from the pressure gauge 60, which indicates the interface pressure in the expansion chamber over time; and controlling the flow rate of the vacuum pump 40 using the controller 50 to maintain the interface pressure at the set pressure. The set pressure may be a pressure to be maintained at a specific time, e.g.,The first interface pressure and / or the second interface pressure mentioned above. A feedback loop is thus provided between the pressure gauge 60 and the controller 50, allowing the controller to continuously adjust the flow rate of the vacuum pump 40 to maintain the interface pressure at the set pressure (e.g., the pressure to be maintained at a given time, such as either the first interface pressure or the second interface pressure). Therefore, if the pressure rises above the set pressure, the pressure signal received by the controller causes the controller to adjust the pump flow rate so that the pressure is reduced to the set value. Conversely, if the pressure falls below the set pressure, the pressure signal received by the controller causes the controller to adjust the pump flow rate so that the pressure is increased to the set value.This type of control can be used to maintain a set pressure in the expansion chamber while mass analysis is performed by the mass spectrometer. The same interface pressure can thus be maintained over time throughout the mass analysis of a sequence of samples.

[0036] In some of the foregoing embodiments, where the first and second flows provide first and second interface pressures corresponding to the first and second operating conditions, the first and second pressures may be essentially the same pressure, which may be the set pressure used by the controller in a feedback loop with the pressure gauge to continuously adjust the vacuum pump flow rate to maintain the interface pressure at the set pressure under both the first and second operating conditions. In some of the foregoing embodiments, where the first and second flows provide first and second interface pressures corresponding to the first and second operating conditions, the first and second pressures may be different pressures.In this case, the first interface pressure can be a first set pressure used by the controller in the feedback loop with the pressure gauge to continuously adjust the flow rate of the vacuum pump in order to maintain the interface pressure at the first set pressure under the first operating condition, and the second interface pressure can be a second set pressure used by the controller in the feedback loop with the pressure gauge to continuously adjust the flow rate of the vacuum pump in order to maintain the interface pressure at the second set pressure under the second operating condition. Thus, some embodiments of the invention provide for the provision of a pressure gauge in the expansion chamber.The use of the pressure gauge may include the following steps: (i) measuring the interface pressure using the pressure gauge and (ii) providing signals representative of the measured pressure to the controller, wherein (iii) the controller compares the measured pressure with a set pressure, and if there is a difference between the measured and the set pressure, the controller adjusts the flow rate of the vacuum pump to reduce the difference between the measured pressure and the set pressure, wherein steps (i) to (iii) are repeated in a feedback loop to keep the interface pressure essentially at the set pressure.

[0037] In some embodiments, a user can override the automatic control and the set interface pressure provided by the computer-aided control software. Thus, at least for a period of time, the automatic regulation of the pump flow rate by the controller can be overridden by the user, allowing the flow rate to be set manually. Typically, the user inputs the pump flow rate directly into the controller. For example, in some such embodiments, a user can adjust the vacuum pump flow rate, and therefore the interface pressure, to optimize the detection sensitivity for one or more elements of interest. This allows the user to fine-tune the vacuum pump flow rate for a period of time to optimize detection sensitivity.The user can thus adjust the vacuum pump's flow rate based on observing the effect such a setting has on the detection sensitivity of one or more elements in a mass spectrum generated on the spectrometer. In such embodiments, the control unit is preferably connected to a visual display or monitor 70 (i.e., a VDU (Visual Display Unit)) so that certain parameters, such as the pump's flow rate, e.g., the pump speed and / or interface pressure, are displayed to the user. The displayed pump flow rate is, at least initially, preferably the automatically controlled flow rate, i.e., the flow rate determined by the automatic control system. The VDU 70 can also display other information to the user, such as one or more of the operating or sampling (plasma) conditions, sample identification, mass spectral data, etc.In some such embodiments, an "override" mode can be implemented. Using a graphical user interface (GUI) displayed on the VDU and an input device, such as a mouse or keyboard, the user can instruct the controller to set a specific vacuum pump flow rate and / or interface pressure. For example, the GUI can provide an instrument control window that may contain a slider which the user can move with an input device, such as a mouse, to adjust the pump flow rate (speed). The slider can be moved to any point on a scale from 0 to 100% of the maximum pump speed in the instrument control window and is thus used to regulate the pump speed and flow rate within a defined range to achieve the interface pressure of interest.Using the invention, it may be possible to control the interface pressure continuously or quasi-continuously from 0.1 to 200 mbar, preferably 0.1 to 100 mbar and more preferably 0.1 to 10 mbar by regulating the flow rate of the backing pump.

[0038] In some embodiments, the GUI can be used by the user to input a selection of an operating condition (e.g., the plasma temperature or the power supplied to the ICP torch) and / or an identification of one or more elements of interest to be mass-analyzed by the spectrometer. In this way, the controller can automatically control the vacuum pump flow rate to optimize the detection sensitivity for one or more elements to be analyzed, depending on the selected operating condition. If the user inputs an identification of one or more elements of interest to be mass-analyzed, the controller can automatically control the vacuum pump flow rate in a fine-tuning mode to optimize the detection sensitivity for the one or more elements. In such embodiments, the controller can be programmed to do this (e.g.,(through their software), to set different vacuum pump flow rates for different elements, each element having its own flow rate setting that optimizes the detection sensitivity for that element under the selected operating (plasma) conditions, and a pump flow rate is thereby set by the controller according to the element(s) of interest specified by the user.

[0039] In another aspect, as an alternative to automatic control of the pump flow rate and / or interface pressure, a method for operating a mass spectrometer vacuum interface is provided, wherein the vacuum interface comprises an evacuated expansion chamber downstream of a plasma ion source at atmospheric or relatively high pressure, wherein the expansion chamber has a first aperture that interfaces with the plasma ion source to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump.to provide an interface pressure in the chamber; wherein the method comprises the use of a controller for controlling the flow rate of the interface vacuum pump to control the interface pressure, wherein a user inputs a specific flow rate of the vacuum pump and / or a specific interface pressure into the controller and the controller adjusts the flow rate of the vacuum pump and / or a specific interface pressure according to the input, for example to optimize the detection sensitivity for one or more elements subjected to mass analysis by the mass spectrometer. Preferably, the controller is connected to a visual display unit (VDU) so that the pump flow rate and / or the interface pressure are displayed to the user, wherein the user instructs the controller by means of a graphical user interface (GUI) displayed on the VDU and an input device.to set a specific flow rate of the vacuum pump and / or a specific interface pressure. The features of the invention described above are also applicable to the further aspect. For example, if the user has entered the flow rate of the pump and / or the interface pressure to be set by the controller, the controller and the pressure gauge maintain the set pressure by means of the described feedback method.

[0040] Similarly, the further aspect also provides a device for operating a mass spectrometer vacuum interface, comprising: a plasma ion source for generating a plasma at atmospheric or relatively high pressure; an evacuated expansion chamber downstream of the plasma ion source, the expansion chamber having a first aperture that interfaces with the plasma ion source to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture to skim off the expanding plasma to form a skimmed expanding plasma; the expansion chamber being pumped by an interface vacuum pump to provide an interface pressure in the expansion chamber;and a controller configured to control the flow rate of the vacuum pump, wherein a user can input a specific vacuum pump flow rate and / or interface pressure into the controller, and the controller adjusts the vacuum pump flow rate and / or interface pressure according to the input, for example, to optimize the detection sensitivity for one or more elements subjected to mass analysis by the mass spectrometer. Preferably, the controller is connected to a visual display unit (VDU) so that the pump flow rate and / or interface pressure is displayed to the user, and wherein the user can instruct the controller to adjust a specific vacuum pump flow rate and / or interface pressure by means of a graphical user interface (GUI) displayed on the VDU and an input device.

[0041] In some embodiments, the data acquired by the mass spectrometer, optionally after processing by the controller, can be used to adjust the vacuum pump flow rate to optimize the spectrometer's detection sensitivity for at least one element. The controller can acquire data from the mass spectrometer and optionally process it to generate a mass spectrum. For one or more elements of interest, the controller can determine the signal intensity (at the detector) from the data for an initial interface vacuum pump flow rate. The controller can then adjust the interface vacuum pump flow rate, acquire data from the mass spectrometer again, and determine the signal intensity again.This process can be repeated until an optimal interface vacuum pump flow rate (and thus an optimal interface pressure) is found that corresponds to a maximum signal intensity for the one or more elements. The controller can then maintain the optimal interface vacuum pump flow rate for measuring the one or more elements. The process can be repeated for one or more different elements to find a specific optimal interface vacuum pump flow rate for each element.

[0042] Advantageously, the invention enables an increase in the sensitivity of the mass spectrometer through a simple regulation of the interface pressure for different sampling conditions, e.g., hot or cold plasma or different measurement modes. Thus, an ICP mass spectrometer is provided in which the regulation of the backing pump flow rate is used to optimize the interface pressure in order to achieve the best sensitivity of the instrument under various experimental conditions.

[0043] In view of the foregoing disclosure, it is understood that embodiments of the invention can be provided in accordance with the following sections: 1) A method for operating a vacuum interface of a mass spectrometer, wherein the vacuum interface comprises an evacuated expansion chamber downstream of a plasma ion source at atmospheric or relatively high pressure, the expansion chamber having a first aperture that interfaces with the plasma ion source to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump to provide an interface pressure in the chamber;wherein the method comprises the use of a controller for automatically controlling the throughput of the interface vacuum pump to control the interface pressure depending on one or more operating modes of the spectrometer in order to optimize the detection sensitivity for one or more elements subjected to mass analysis by the mass spectrometer. 2) Method according to Section 1, wherein the throughput of the interface vacuum pump is controlled depending on one or more operating conditions of the plasma ion source and / or one or more elements of interest that are to be mass-analyzed by the spectrometer. 3) Method according to Section 2, wherein the plasma ion source is an inductively coupled plasma (ICP) ion source and the one or more operating conditions include a plasma temperature, a plasma torch position and / or a plasma gas flow. 4) Method according to any of the preceding sections, wherein the control comprises a computer and associated control electronics which have an interface with the interface vacuum pump. 5) Method according to any of the preceding sections, wherein the interface vacuum pump is a backing pump for a high vacuum pump which pumps a high vacuum region of the mass spectrometer, and the interface pressure in the chamber is controlled to be in a range of 0.1 to 10 mbar. 6) Method according to any of the preceding sections, comprising acquiring data from the mass spectrometer at the controller and using the data to adjust the throughput of the vacuum pump in order to optimize the detection sensitivity of the spectrometer for at least one element. 7) A method according to any of the preceding sections, further comprising changing the operating conditions of the plasma ion source from a first operating condition to a second operating condition or vice versa, and in each case automatically controlling the throughput of the interface vacuum pump from a first throughput when operating the plasma under the first operating condition to a second throughput when operating the plasma under the second operating condition, wherein the first and second throughputs are different from each other. 8) Method according to Section 7, wherein the first operating condition is a hot plasma condition and the second operating condition is a cold plasma condition. 9) Method according to Section 7 or 8, wherein the first throughput provides a first interface pressure in the interface vacuum stage to optimize the detection sensitivity for at least one element of the sample to be analyzed using the first operating condition, and the second throughput provides a second interface pressure to optimize the detection sensitivity for at least one element that is mass analyzed using the second operating condition. 10) Method according to Section 9, wherein the at least one element whose detection sensitivity is optimized under the first operating condition differs from the at least one element whose detection sensitivity is optimized under the second operating condition. 11) Method according to Section 9 or 10, wherein the first and second interface pressures are controlled so that they are substantially the same. 12) Method according to any of the preceding sections, wherein more than two different operating conditions are provided, each of which has a respective vacuum pump throughput set by the control. 13) A method according to any of the preceding sections, further comprising providing a pressure gauge in the expansion chamber, (i) measuring the interface pressure using the pressure gauge, (ii) providing signals representative of the measured pressure to the controller, and (iii) comparing the measured pressure with a set pressure using the controller, and if the controller determines that there is a difference between the measured and the set pressure, the controller adjusts the flow rate of the vacuum pump to reduce the difference between the measured pressure and the set pressure, with steps (i) - (iii) being repeated in a feedback loop to keep the interface pressure substantially at the set pressure. 14) Method according to any of the preceding sections, wherein the control is connected to a visual display unit (VDU) so that the pump flow rate, which is preferably initially controlled automatically, and / or the interface pressure are displayed to a user, and wherein the user overrides the automatic control via a graphical user interface (GUI) displayed on the VDU and an input device and manually sets a specific vacuum pump flow rate and / or a specific interface pressure. 15) Device for operating a vacuum interface of a mass spectrometer, comprising: a plasma ion source for generating a plasma at atmospheric or relatively high pressure; an evacuated expansion chamber downstream of the plasma ion source, the expansion chamber having a first aperture that interfaces with the plasma ion source to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture to skim off the expanding plasma to form a skimmed expanding plasma; the expansion chamber being pumped by an interface vacuum pump to provide an interface pressure in the expansion chamber; and a controller configured to automatically control the flow rate of the vacuum pump depending on one or more operating modes of the spectrometer. 16) Device according to Section 15, wherein the control is configured to automatically control the throughput of the vacuum pump depending on a plasma condition and / or a measurement mode. 17) Device according to Section 15 or 16, wherein the plasma ion source is an inductively coupled plasma (ICP) ion source. 18) Device according to any of sections 15 to 17, wherein the control comprises a computer and associated control electronics which has an interface with the interface vacuum pump. 19) Device according to any of sections 15 to 18, wherein the flow rate of the vacuum pump can be continuously or quasi-continuously adjusted by the control over a range of flow rates. 20) Device according to any of sections 15 to 19, wherein the interface vacuum pump is a backing pump for a high vacuum pump which pumps a high vacuum region of the mass spectrometer. 21) Device according to any of sections 15 to 20, wherein the control is configured to automatically adjust the throughput of the interface vacuum pump from a first throughput when operating the plasma ion source under a first operating condition to a second throughput when operating the plasma ion source under a second operating condition, wherein the first and second throughputs are different from each other. 22) Device according to Section 21, wherein the first operating condition and the second operating condition differ in the temperature of the plasma, wherein preferably the first operating condition is a hot plasma condition and the second operating condition is a cold plasma condition. 23) Device according to Section 21 or 22, wherein the first throughput provides a first interface pressure in the interface vacuum stage to optimize the detection sensitivity for at least one element of the sample to be analyzed using the first operating condition, and the second throughput provides a second interface pressure to optimize the detection sensitivity for at least one element that is mass analyzed using the second operating condition. 24) Device according to Section 23, wherein the at least one element whose detection sensitivity is optimized under the first operating condition differs from the at least one element whose detection sensitivity is optimized under the second operating condition. 25) Device according to section 23 or 24, wherein the control system controls the first and second interface pressures so that they are substantially equal. 26) Device according to any of sections 15 to 25, wherein a manometer is located in the expansion chamber and a feedback loop is provided between the manometer and the control so that the control can continuously adjust the flow rate of the vacuum pump to maintain the interface pressure at a set pressure. 27) Device according to any of sections 15 to 26, wherein the control is connected to a visual display unit (VDU) so that the pump flow rate and / or the interface pressure is displayed to a user, and wherein the user can instruct the control to set a specific vacuum pump flow rate and / or interface pressure by means of a graphical user interface (GUI) displayed on the VDU and an input device. 28) Method for operating a vacuum interface of a mass spectrometer, wherein the vacuum interface comprises an evacuated expansion chamber downstream of a plasma ion source at atmospheric or relatively high pressure, the expansion chamber having a first aperture that interfaces with the plasma ion source to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump to provide an interface pressure in the chamber;wherein the method comprises using a controller to control the throughput of the interface vacuum pump to control the interface pressure, wherein a user inputs a specific vacuum pump throughput and / or a specific interface pressure into the controller and the controller sets the vacuum pump throughput and / or a specific interface pressure according to the input. 29) Method according to Section 28, wherein the controller is connected to a visual display unit (VDU) so that the pump flow rate and / or the interface pressure is displayed to the user, wherein the user instructs the controller to set a specific vacuum pump flow rate and / or interface pressure by means of a graphical user interface (GUI) displayed on the VDU and an input device. 30) Method according to section 28 or 29, wherein a pressure gauge is located in the expansion chamber and a feedback loop is provided between the pressure gauge and the control so that the control can maintain the interface pressure at the set pressure. 31) Device for operating a mass spectrometer vacuum interface, comprising: a plasma ion source for generating a plasma at atmospheric or relatively high pressure; an evacuated expansion chamber downstream of the plasma ion source, the expansion chamber having a first aperture that interfaces with the plasma ion source to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture to skim off the expanding plasma to form a skimmed expanding plasma; the expansion chamber being pumped by an interface vacuum pump to provide an interface pressure in the expansion chamber;and a controller configured to control the flow rate of the vacuum pump, wherein a user can input a specific flow rate of the vacuum pump and / or a specific interface pressure into the controller and the controller sets the flow rate of the vacuum pump and / or a specific interface pressure according to the input.

Claims

Method for operating a vacuum interface (3) of a mass spectrometer (10), wherein the vacuum interface (3) comprises an evacuated expansion chamber downstream of a plasma ion source (6), the plasma ion source (6) being located at a pressure between atmospheric pressure and 100 mbar, the expansion chamber having a first aperture interface with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump (40),to provide an interface pressure in the expansion chamber; wherein the method comprises using a controller (50) to automatically control the flow rate of the interface vacuum pump (40) to control the interface pressure depending on operating modes of the mass spectrometer (10), wherein the flow rate of the interface vacuum pump (40) is controlled by changing the pump speed by varying the operating voltage and / or the operating current of the interface vacuum pump (40), and the flow rate of the interface vacuum pump (40) is controlled depending on one or more operating conditions of the plasma ion source (6) and / or one or more elements of interest to be mass-analyzed by the mass spectrometer (10), wherein the controller (50) is connected to a visual display unit (VDU),so that the initially automatically controlled flow rate of the interface vacuum pump (40) and / or the interface pressure are displayed to a user, and wherein the user overrides the automatic control (50) via a graphical user interface (GUI) displayed on the VDU and an input device (72) and manually sets a specific flow rate of the vacuum pump (40) and / or a specific interface pressure. Method according to claim 1, wherein the plasma ion source (6) is an inductively coupled plasma (ICP) ion source and the one or more operating conditions comprise a plasma temperature, a plasma torch position and / or a plasma gas flow. Method according to one of the preceding claims, wherein the control (50) comprises a computer and associated control electronics which has an interface with the interface vacuum pump (40). Method according to one of the preceding claims, wherein the interface vacuum pump (40) is a backing pump for a high vacuum pump which pumps a high vacuum region of the mass spectrometer (10), and the interface pressure in the expansion chamber is controlled such that it is in a range of 0.1 to 10 mbar. Method according to one of the preceding claims, comprising acquiring data from the mass spectrometer (10) at the controller (50) and using the data to adjust the throughput of the interface vacuum pump (40) in order to optimize the detection sensitivity of the mass spectrometer (10) for at least one element. A method according to one of the preceding claims, further comprising changing the operating conditions of the plasma ion source (6) from a first operating condition to a second operating condition or vice versa, and in each case automatically controlling the throughput of the interface vacuum pump (40) from a first throughput when operating the plasma under the first operating condition to a second throughput when operating the plasma under the second operating condition, wherein the first and the second throughput are different from each other. Method according to claim 6, wherein the first operating condition is a hot plasma condition and the second operating condition is a cold plasma condition. Method according to claim 6 or 7, wherein the first throughput provides a first interface pressure in the vacuum interface (3) to optimize the detection sensitivity for at least one element of the sample to be analyzed using the first operating condition, and the second throughput provides a second interface pressure to optimize the detection sensitivity for at least one element that is mass analyzed using the second operating condition. Method according to claim 8, wherein the at least one element whose detection sensitivity is optimized under the first operating condition differs from the at least one element whose detection sensitivity is optimized under the second operating condition. Method according to claim 8 or 9, wherein the first and second interface pressures are controlled such that they are substantially the same. Method according to one of the preceding claims, wherein more than two different operating conditions are provided, each of which has a respective vacuum pump throughput that is set by the control (50). A method according to any of the preceding claims, further comprising providing a pressure gauge (60) in the expansion chamber, (i) measuring the interface pressure using the pressure gauge (60), (ii) providing signals representative of the measured pressure to the controller (50), and (iii) comparing the measured pressure with a set pressure using the controller (50), and if the controller (50) determines that there is a difference between the measured and the set pressure, the controller (50) adjusts the flow rate of the interface vacuum pump (40) to reduce the difference between the measured pressure and the set pressure, wherein steps (i) - (iii) are repeated in a feedback loop to keep the interface pressure substantially at the set pressure. Device for operating a vacuum interface of a mass spectrometer (10), comprising: a plasma ion source (6) for generating a plasma between atmospheric pressure and 100 mbar; an evacuated expansion chamber downstream of the plasma ion source (6), the expansion chamber having a first aperture that interfaces with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture to skim off the expanding plasma to form a skimmed expanding plasma; the expansion chamber being pumped by an interface vacuum pump (40) to provide an interface pressure in the expansion chamber; and a controller (50) configured tothat it automatically controls the throughput of the interface vacuum pump (40) depending on the operating modes of the mass spectrometer (10) by changing the pump speed by varying the operating voltage and / or the operating current of the interface vacuum pump (40), wherein the throughput of the interface vacuum pump (40) is controlled depending on one or more operating conditions of the plasma ion source (6) and / or one or more elements of interest to be mass-analyzed by the mass spectrometer (10), wherein the controller (50) is connected to a visual display unit (VDU) so that the throughput of the interface vacuum pump (40) and / or the interface pressure are displayed to a user, and wherein the user can instruct the controller (50) by means of a graphical user interface (GUI) displayed on the VDU and an input device (72),to set a specific throughput of the interface vacuum pump (40) and / or a specific interface pressure. Device according to claim 13, wherein the controller (50) is configured to automatically control the throughput of the interface vacuum pump (40) depending on a plasma condition and / or a measurement mode. Device according to claim 13 or 14, wherein the plasma ion source (6) is an inductively coupled plasma (ICP) ion source. Device according to one of claims 13 to 15, wherein the control (50) comprises a computer and associated control electronics which has an interface with the interface vacuum pump (40). Device according to one of claims 13 to 16, wherein the throughput of the interface vacuum pump (40) can be continuously or quasi-continuously adjusted by the control (50) over a range of throughput speeds. Device according to one of claims 13 to 17, wherein the interface vacuum pump (40) is a backing pump for a high vacuum pump that pumps a high vacuum region of the mass spectrometer (10). Device according to one of claims 13 to 18, wherein the control (50) is configured to automatically adjust the throughput of the interface vacuum pump (40) from a first throughput when operating the plasma ion source (6) under a first operating condition to a second throughput when operating the plasma ion source (6) under a second operating condition, wherein the first and the second throughput are different from each other. Device according to claim 19, wherein the first operating condition and the second operating condition differ in the temperature of the plasma, wherein preferably the first operating condition is a condition with hot plasma and the second operating condition is a condition with cold plasma. Device according to claim 19 or 20, wherein the first throughput provides a first interface pressure in the vacuum interface (3) to optimize the detection sensitivity for at least one element of the sample to be analyzed using the first operating condition, and the second throughput provides a second interface pressure to optimize the detection sensitivity for at least one element that is mass analyzed using the second operating condition. Device according to claim 21, wherein the at least one element whose detection sensitivity is optimized under the first operating condition differs from the at least one element whose detection sensitivity is optimized under the second operating condition. Device according to claim 21 or 22, wherein the control (50) controls the first and the second interface pressure such that they are substantially equal. Device according to claim 13 or 23, wherein a manometer (60) is located in the expansion chamber and a feedback loop is provided between the manometer (60) and the controller (50) so that the controller (50) can continuously adjust the flow rate of the interface vacuum pump (40) to maintain the interface pressure at a set pressure. Method for operating a vacuum interface of a mass spectrometer (10), wherein the vacuum interface comprises an evacuated expansion chamber downstream of a plasma ion source (6), the plasma ion source (6) being located at a pressure between atmospheric pressure and 100 mbar, the expansion chamber having a first aperture that interfaces with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture from the plasma to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump (40) to provide an interface pressure in the expansion chamber; wherein the method comprises using a controller (50) to control the flow rate of the interface vacuum pump (40) to control the interface pressure.wherein the flow rate of the interface vacuum pump (40) is controlled by changing the pump speed by varying the operating voltage and / or the operating current of the interface vacuum pump (40), wherein a user inputs a specific flow rate of the interface vacuum pump (40) and / or a specific interface pressure into the controller (50) and the controller (50) sets the flow rate of the interface vacuum pump (40) and / or a specific interface pressure according to the input, wherein the controller (50) is connected to a visual display unit (VDU) so that the flow rate of the interface vacuum pump (40) and / or the interface pressure is displayed to the user, wherein the user instructs the controller (50) to set a specific flow rate of the interface vacuum pump (40) and / or a specific interface pressure by means of a graphical user interface (GUI) displayed on the VDU and an input device (72). Method according to claim 25, wherein a manometer (60) is located in the expansion chamber and a feedback loop is provided between the manometer (60) and the control (50) so that the control (50) can maintain the interface pressure at the set pressure. Device for operating a mass spectrometer vacuum interface, comprising: a plasma ion source (6) for generating a plasma, wherein the plasma ion source (6) is located at a pressure between atmospheric pressure and 100 mbar; an evacuated expansion chamber downstream of the plasma ion source (6), wherein the expansion chamber has a first aperture that interfaces with the plasma ion source (6) to form an expanding plasma downstream of the first aperture, and a second aperture downstream of the first aperture to skim off the expanding plasma to form a skimmed expanding plasma; wherein the expansion chamber is pumped by an interface vacuum pump (40) to provide an interface pressure in the expansion chamber;and a controller (50) configured to control the flow rate of the interface vacuum pump (40), the controller (50) being connected to a visual display unit (VDU) for displaying a graphical user interface (GUI) and to an input device (72), the flow rate of the interface vacuum pump (40) being controlled by changing the pump speed by varying the operating voltage and / or the operating current of the interface vacuum pump (40), a user being shown a specific flow rate of the interface vacuum pump (40) and / or a specific interface pressure on the visual display unit, and the user being able to input the flow rate of the interface vacuum pump (40) and / or the interface pressure into the controller (50) by means of the input device (72), and the controller (50) setting the flow rate of the interface vacuum pump (40) and / or a specific interface pressure according to the input.

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